As one of conventional systems of estimating a position of magnetic pole of an electric motor, an induced voltage proportional to a rotational speed of the motor is computed based on an input voltage and an input current of the motor. A magnetic pole position is estimated based on the obtained induced voltage. This estimation system is widely used. Furthermore, an AC signal is superposed on a voltage command value, so that an estimated current is analyzed by a First Fourier Transform analysis, whereby a rotational speed and a magnetic pole position of the motor are estimated. However, an accurate estimation cannot be obtained in a very low speed range although this method can achieve a sufficient accuracy in a high speed range having less amount of information about an induced voltage.
In view of the above problem, several systems have been proposed in which a sensing AC signal that is not involved in a drive frequency is applied to a motor so that a rotor position is estimated from a relationship between voltage and current. However, application of a sensing signal necessitates a particular signal generator, resulting in a complicate control manner.
In further another system differing from the above-described ones, a position of magnetic pole is estimated using a current of high frequency components or carrier frequency components both contained in an inverter output without application of a particular sensing signal. In the former, inductance is computed from high frequency current contained in PWM inverter output. The magnetic pole position is estimated on the basis of the obtained inductance. In the latter, a phase difference of 120° is caused in carrier signals of the PWM inverter between two of three phases U, V and W, so that a carrier frequency component voltage and a carrier frequency component current both other than a drive frequency are generated. Based on a supposition that voltage is constant during a carrier period, the magnetic pole position is estimated using only the carrier frequency component current.
In the above-described magnetic pole position estimating system, a high frequency current flowing due to a high frequency voltage is a disturbance against a voltage of fundamental components of the inverter output. However, since the carrier frequency is sufficiently high relative to a rotational speed of the motor, the aforesaid high frequency current is not a disturbance against torque. Furthermore, a low pass filter or the like needs to be added to a current feedback value in the magnetic pole position estimation, whereupon the responsiveness of the control system can advantageously be improved.
However, from the viewpoint of practical use, the magnitude of the high frequency current depends upon parameters of the motor and accordingly, influences of the high frequency current differ depending upon motors to be used, whereupon it would be difficult to apply the magnetic pole position estimating system to various types of systems in a versatile manner.